The invention directed to improvements in piezoelectric actuators, in particular for actuating control valves or injection valves of internal combustion engines in motor vehicles, having a piezoelectric actuator body, in particular in the form of a multi-layer laminate of stacked layers of piezoelectric material and between them metal or electrically conductive layers acting as electrodes, in which the actuator body is surrounded by a module wall, while maintaining an interstice or intermediate chamber that is divided into an upper chamber and lower chamber by an axially movable partition located transversely on the actuator head.
One such piezoelectric actuator disclosed in German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH.
As is well known, piezoelectric actuators can for instance be used for injection valves of a vehicle motor and in brake systems with anti-lock and traction control systems.
Such injection valves equipped with piezoelectric actuators have an injection nozzle controlled by a tappetlike closure device. An operative face toward the nozzle is disposed on the tappet and is acted upon by the pressure of the fuel supplied to the nozzle; the pressure forces seek to urge the tappet in the opening direction of the closure device. The tappet protrudes with a plungerlike end, whose cross section is larger than the aforementioned operative face, into a control chamber. The pressure effective there seeks to urge the tappet in the closing direction of the closure device. The control chamber communicates with the fuel supply, which is at a high pressure, via an inlet throttle and with a fuel return line that has only low pressure, via an outlet valve that is throttled as a rule or is combined with an outlet throttle. When the outlet valve is closed, a high pressure prevails in the control chamber, by which the tappet is moved in the closing direction of the closure device, counter to the pressure on its operative face toward the nozzle, or is kept in the closing position. Upon opening of the outlet valve, the pressure in the control chamber drops; the magnitude of the drop in pressure is determined by the size of the inlet throttle and by the throttle resistance of the opened outlet valve, or the outlet throttle combined with it. As a result, the pressure in the control chamber decreases when the outlet valve is opened, in such a way that the tappet is moved in the opening direction of the closure device, or held in the open position, by the pressure forces that are operative on its operative face toward the nozzle.
In comparison with electromagnetically actuated injection valves, piezoelectric actuators can switch faster. However, in the design of a piezoelectric actuator, it must be noted that internal losses in the piezoelectric body of the actuator cause lost heat, which has to be dissipated so that the actuator will not overheat. Since the ceramic materials of the piezoelectric ceramic have poorer heat conductivity, the dissipation inside the actuator body, which substantially comprises ceramic material, is unfavorable.
Cooling the actuator with a liquid coolant, such as fuel, water, motor oil and the like, is unfavorable, first because of the risk of a short circuit from the water component that is contained both in the fuel and in motor oil, and second because the actuator module is more expensive because of complicated seals, which must preclude the coolant used from escaping from the actuator module, especially when the actuator becomes heated.
It is therefore the object of the invention to make a piezoelectric actuator of this kind possible in such a way that cooling can be done during its operation without a liquid coolant, such as motor oil, water or fuel; that the piezoelectric actuator can be installed simply; and that no special seals, as in liquid cooling, are needed.
To attain these objects, a piezoelectric actuator according to the invention is characterized in that the upper and/or lower chamber, which surrounds the actuator body inside the module wall, experiences a flow through it of cooling air for cooling the actuator body.
A piezoelectric actuator designed according to the invention can use compressed air, which is available always in commercial or utility vehicles, for cooling the actuator body. Even small quantities of air flowing through the actuator module suffice to achieve adequate actuator cooling, and the engineering expense is slight, except for the hose connection to supply the cooling air. The functional safety of the piezoelectric actuator according to the invention is great, since liquid coolant cannot escape from the actuator module.
In one embodiment, a cooling air stream is delivered to the lower chamber, which is located below the partition resting crosswise on the actuator head. In this embodiment, by means of an elastomer jacket or shrink-fit hose sheathing the actuator body and its flow connections, care is taken to prevent any moisture contained in the cooling air stream from causing damage or short circuits in the interior of the actuator module. In this case, special provisions for sealing off the upper chamber in moisture-proof fashion from the lower chamber through which the cooling air stream flows are unnecessary.
In an alternative embodiment, only the upper chamber above the actuator head has a cooling air stream flowing through it. In that case, care is taken to assure that the upper chamber is sealed off from the lower chamber in moisture-proof fashion. In this type of embodiment, the engine heat does not reach the actuator body, either. To that end, the partition resting crosswise on the actuator head is sealed off on its periphery from the actuator wall by an O-ring. In addition, as in the first embodiment, in which the cooling air stream flows through the lower chamber, the actuator body and its connection lines are sheathed by an elastomer jacket or shrink-fit hose. Furthermore, an additional elastomer sealing ring can be provided for sealing purposes above the partition; it is fixed to the module wall on its periphery by radial bumps or an annular bead. In the interior region, sealing of the upper chamber from the lower chamber can be attained by an adhesive layer with good thermal conductivity.
In a further variant, a cooling baffle can additionally be joined to the partition in a manner assuring good thermal conductivity and can assure additional cooling, since it is located in the upper chamber through which the cooling air flows.
Instead of an elastic seal of elastomer material, a steel diaphragm can also be used, which at the same time has good thermal conductivity and assures additional heat dissipation from the lower chamber of the actuator module to the upper chamber, through which the cooling air flows. Such a steel diaphragm can be joined tightly and thermally conductively to the module wall by screwing, using a screw ring.
Especially advantageously, the invention can be employed in commercial vehicles for Diesel fuel injection in a common rail injector system. As noted, in commercial vehicles compressed air is available, and a slight quantity of it can be utilized to cool the piezoelectric actuator.
Further advantageous characteristics of a piezoelectric actuator designed according to the invention will become even more apparent from the description of various exemplary embodiments, taken in conjunction with the drawing.